1. Field of the Invention
This invention relates to a silicone rubber composition which on heat curing provides silicone rubber serving as an improved high-voltage electrical insulator.
2. Prior Art
In general, high-voltage electrical insulating materials for use as insulators for power transmission lines are of porcelain or glass. In an environment subject to pollution as in seaside areas and industrial areas, there is a tendency that dust, salts and mist attach to the surface of high-voltage electrical insulators, causing leakages of current and dry band discharge leading to flashover.
In order to eliminate the drawbacks of porcelain and glass insulators, a number of proposals have been made. For example, U.S. Pat. No. 3,511,698 discloses a weather resistant high-voltage electrical insulator comprising a member of a curable resin and a platinum catalyst-containing organopolysiloxane elastomer. JP-A 198604/1984 corresponding to U.S. Pat. No. 4,476,155 proposes a one-part room temperature curable organopolysiloxane composition which is applied to the outer surface of an electrical insulator of glass or porcelain so that the electrical insulator may maintain its high electrical properties even in the presence of outdoor stresses such as moisture, air pollution and ultraviolet radiation.
JP-B 35982/1978 corresponding to U.S. Pat. No. 3,965,065 and JP-A 209655/1992 corresponding to U.S. Pat. No. 5,369,161 disclose that a silicone rubber composition with improved electrical insulation is obtained by heating a mixture of an organopolysiloxane capable of heat curing into silicone rubber and aluminum hydrate at temperatures above 100.degree. C. for more than 30 minutes.
However, some silicone rubber materials used in the prior art techniques mentioned above are not yet fully satisfactory in high-voltage electrical insulation. Some other silicone rubber materials require an undesirably long time for curing. Also it is believed in the prior art that if platinum catalysts are used in curing silicone rubber materials, the resulting silicone rubbers are deteriorated in electrical insulation. For avoiding such adverse impact, it has been often attempted to use organic peroxides instead of the addition curing type using platinum catalysts. However, since compositions of the organic peroxide crosslinking type are based on a high-viscosity raw rubber like organopolysiloxane, a sheeting or preforming step of milling the composition by roll mills is necessary in order that the composition be ready for supply to molding machines. This sheeting or preforming step requires more amounts of labor than the injection molding step and thus forms a bar toward labor saving.